1. Field of the Invention
The present invention relates to a novel titanium-based composite oxide useful as an active material of a lithium second battery and a lithium secondary battery using the same.
2. Description of the Related Art
Lithium secondary batteries have progressed as electric power supplies for cellular phones and laptop computers because the energy densities of lithium secondary batteries are high; however, with the miniaturization and achievement of lightweightness of portable terminal devices ascribable to the recent progress of IT technology, batteries as the electric power supplies of such devices have come to be required to be further reduced in size and to be higher in capacity. Additionally, in a manner making the most of the high energy densities of lithium secondary batteries, lithium secondary batteries come to attract attention as electric power supplies for use in electric automobiles and hybrid automobiles and as power storage-type power supplies.
With respect to the negative electrode materials of lithium batteries, carbon-based negative electrodes have hitherto been commonly used; the lithium secondary batteries using carbon-based negative electrodes are characterized in that the voltage and the energy density at the time of discharge are high. However, because the potential of the negative electrode is low, when rapid charge is performed, lithium metal is deposited to increase the risk of causing internal short circuiting, and further, there is an inherent risk such that such internal short circuiting leads to the occurrence of flame. Accordingly, there have been investigated lithium batteries in which although the energy density is decreased, the heat generation at the time of internal short circuiting is reduced by using a high-potential negative electrode, and further by suppressing the decomposition of the electrolyte, the safety is enhanced and the operation life is extended. Among others, Li4Ti5O12 has a potential of 1.5 V with reference to lithium, is free from the volume change at the time of charge/discharge and is extremely satisfactory in cycle properties, and hence coin batteries using Li4Ti5O12 have been put into practical use.
However, the theoretical capacity of Li4Ti5O12 is 175 mAh/g, which leads to a drawback such that the electric capacity of Li4Ti5O12 is as small as approximately half the electric capacity of carbon, which is commonly used as a negative electrode material, and the energy densities of lithium secondary batteries using Li4Ti5O12 are also small. Therefore, from the viewpoint of the safety and long operating life, a negative electrode material which has a potential of 1.0 to 1.5 V with reference to lithium and a large electric capacity has been demanded.
Under such circumstances, the titanium oxide obtained by using as a starting material K2Ti4O9 or Na2Ti3O7 having layered structure and by performing proton exchange and thermal dehydration is referred to as the bronze-structure titanium oxide or TiO2(B), and has a layered structure or a tunnel structure, and hence is attracting attention as an electrode material.
For example, although it has been found that a high charge/discharge capacity of 200 mAh/g or more is obtained by converting the bronze-structure titanium oxide compound into nanoparticles (A. R. Armstrong et al., ADVANCED MATERIALS, 2005, 17, No. 7, pp. 862 to 865), such a compound has a low bulk density and a large specific surface area, and hence the packing density of the electrode is low, the adhesion between the coating film and the current collector tends to be aggravated, and thus such a compound is not necessarily to be claimed as excellent as an active material. On the other hand, the bronze-structure titanium oxide of micron size, obtained by a solid phase method through K2Ti4O9 or Na2Ti3O7 is capable of being reduced in specific surface area and has a firm particle framework, and hence is satisfactory in cycle properties, but is disadvantageously small in charge/discharge capacity (Japanese Patent Laid-Open Nos. 2008-34368 and 2008-117625).
As described above, the electric capacities of conventional lithium secondary batteries are still insufficient, and hence negative electrode materials which are materials large in electric capacity and are capable of maintaining the capacities are demanded.